Tem mode directional coupler having dielectric compensating means



NOV. 11, I D. H. JEQNG TEM MODE DIRECTIONAL COUPLER HAVING DIELECTRIC COMPENSATING MEANS Filed July 25, 1968 2 Sheets-Sheet 1 FIGURE I FIGURE 3 Nov. 11. 1969 o. H. JEQNG 3,478,231

TEM MODE DIRECTIONAL COUPLER HAVING DIELECTRIC COMPENSATING MEANS Filed July 25, 1968 2 Sheets-Sheet 2 INVENTOR DAVID H. JEONG United States Patent ABSTRACT OF. THE DISCLOSURE Annular dielectric elements support the coupling elements of a TEM mode directional coupler ma desired couplingrelationship while'providing the coupler with more uniform transmission line impedance and'higher directivity over a broad frequency range.

BACKGROUND AND SUMMARY-OF THE INVENTION This invention relates to directional couplers, and particularly to high fiequencyTEM mode three db directional couplers.

A three db directional coupler is a device-which transfers half the power from onetransmission line to a'second transmission line in such a manner that the coupled power in the second transmission line flows in one direction only. This can generally be achieved with an odd number of quarter wavelength coupling sections; however, the combination of such couplings provides a large coupler structurewhile the relatively short wavelengths corresponding to frequencies of eight gHz. and above demand that the coupler structure remain small to prevent the introduction of higher order modes. But small structures present higher attenuation and certain connecting difliculties due to the fact that the diameter of a transmission-line of the coupler may be an appreciable portion-of a wavelength. Also,' at these frequencies, great care is"required in impedance matching to provide broad bandwidthand low attenuation.

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as to provide a three db directional coupler. The coupler comprises two line conductors supported between two parallel ground planes by annular dielectric elements having a relative permittivity greater than that of air. These dielectric elements are strategically placed between the ground planes and the diverging portions of the line conductors to enhance the electrical properties of the directional coupler as well as to support the line conductors. To provide a desired coupling factor for such a coupler at high frequencies, the parallelportions of the line conductors are of semicircular cross section with their flat sides disposedopposite one another. Transition of the transmission lines to their semicircular sections is made by "tapering the diverging portions of the line conductors as they'a-pproach the parallel couplingregion. This will normally raise the impedance of these diverging sections gratly,- causing a large discontinuity in the transition from the remote line'conductors to the parallel portions of these conductors. Therefore, two tapered dielectric supports are provided between the tapered diverging portions of the line conductors and the ground planes. Their angle of taper and dielectric constant are so selected to provide a substantially uniform impedance along the tapered diverging portions of the conductors. The dielectric material is chosen such that it provides the correct impedance while filling all of the space between the line conductors and theground planes with dielectric at the inner diameter of the supports. The dielectric material serves the electrical functions of providing a substantially uniform impedance and improving coupler directivity while supporting the parallel center conductors in the coupling position.

DESCRIPTION OF THE DRAWINGS 'FIGURE 1 is a cutaway top view of the preferred embodiment of the invention.

FIGURE 2 is an elevational sectional view of the preferred embodiment along the line AA.

addition, where the auxiliary transmission line of a I quarter wavelength coupling section diverges from'the primary transmission line, an electromagnetic wave is nondirectionally'coupled to the diverging portion of the "auxiliary transmission line. This non-directional electromagnetic *wave has sufiicient voltage to 's'eriously'impair the directivity of the TEM mode coupler at frequencies ranging above one gHz, per second.

One method of reducing the voltage of the non-directional electromagnetic wave..and thereby increasing the directivity of the TEM mode coupler is to place a conductive element near the auxiliary transmission line where it diverges from the primary'transmission lineas disclosed and claimed in Harmons US. Patent 3,204,206, en-

titled High Directivity TEM Mode Coupler, and issued Aug. 31, 1965. The effect of such placement of the conductive element is to reduce the electric field coupling between the diverging portions of the primary and auxiliary transmission lines by grounding a portion of the electric field at the point of divergence. This compensates for the otherwise greater reduction in the magnetic field coupling between the diverging portions of the primary and auxiliary transmission lines and diminishes the voltage of the resultant non-directional electromagnetic wave. However, one disadvantage of this method is that the placement of the conductive element is very critical and tuning is usually required.

In accordance with the illustrated embodiment of this invention, there is provided a single quarter wavelength electric and magnetic field coupling section mounted so FIGURE 3 is an elevational sectional view of the preferred embodiment along the line B-B.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, there is shown a directional coupler having line conductors 10 and 12 with parallel coupling portions 14 mounted in transverse electric a'nd'magnetic field coupling proximity to provide a single quarter wavelength coupling section 18 within parallel ground planes 20 and 22. These parallel portions 14 or line conductors 10 and 12 are equidistant from ground planes 20 and 22 and have semicircular cross sections with" their flat portions closely spaced adjacent to one another. Parallel portions 14 of line conductors 10 and 12 are maintained in this position to provide a desired coupling factor for the directional coupler. The ground planes 20 and 22 are cut backat regions 26 to provide a desired line impedance in parallel coupling portions 14. Line conductors 10 .and 12 are maintained and supported in this coupling position by dielectric compensation supports 16 which are of annular shape and are mounted on ground planes 20 and 22.

The cross sections of diverging portions 24 of the line conductors are reduced by tapering to provide a smooth transition to the parallel portions 14 of semicircular crosssectional shape. This tapering, however, increases the impedance of the line conductors in the tapered diverging region 24.

The annular dielectric compensation supports 16 are mounted on ground plane conductors 20 and 22 near the tapered diverging regions 24 of line conductors 10 and 12 to help maintain a uniform transmission line impedance which is substantially equal to that of the transmission line remote from the directional coupler and to improve .thgLdirectiyity of the directional coupler. These dielectric compensation supports 16 are tapered, being thinner at their outer diameters than at their inner diameters, to provide a constant impedance, substantially equal to that of the line conductors remote from the coupling region, at each point along the tapered diverging regions 24 of fi11 all the space between line conductors and 12'and ground planes and 22. The angle of taper of dielectric supports 16 isselected to provide a substantially uniform impedance along the length of the tapered diverging regions24 of transmission line conductors 10 and 12.

I claim:

1. A TEM mode directional coupler comprising:

at least one planar reference conductor;

first and second electromagnetic wave energy trans- -'mission paths including first and second line conductors spaced a finite distance from said one planar reference conductor;

said first and second line conductors including parallel coupling portions positioned in the same plane in transverse electric and magnetic field coupling proximity with a finite spacing therebetween;

said second line conductor including a first portion which diverges from said first line conductor;

a dielectric medium filling the space between said one planar reference conductor and said line conductors; and

dielectric compensating means having a greater relative permittivity than that of said dielectric medium, said dielectric compensating means supporting said line conductors the finite distance from said one planar reference conductor and being positioned between said one planar reference conductor and the region of divergence of said line conductors to improve at least one electrical parameter of said coupler.

2. A directional coupler as in claim 1 wherein said dielectric compensating means supports said line conductors in a plane substantially parallel to the plane of said one planar reference conductor.

3. A directional coupler as in claim 2 wherein: another planar reference conductor is positioned adjacent and parallel to said one planar reference conductor;

4' said dielectric medium fills the space between said 5. A directional coupler as in claim 4 .wherein:

another electrical parameter is directivity; and

said dielectric compensating means helps to improve the directivity of said coupler.

6. A directional coupler as in claim 5 wherein:

said second line conductor includes a second portion which diverges from said first line conductor, said first and second diverging portions of said second line conductor being located on opposite sides of said coupling portion of said second line conductor; and

said coupler includes additional dielectric compensating means having a greater relative permittivity than that of said dielectric medium, said first-mentioned dielectric compensating means being positioned between said one planar reference conductor and the regions of divergence of said line conductors and said last-mentioned dielectric compensating means being positioned between said other planar reference conductor and the regions of divergence of said line conductors, both of said dielectric compensating means supporting said line conductors substantially equidistant between said planar reference conductors and helping to improve the directivity and the uniformity of transmission line impedance of said coupler.

7. A directional coupler as in claim 6 wherein:

' said line conductors each have cross sections of substantially semicircular shape along the parallel coupling portions, the fiat portions of said cross sections or said line conductors being adjacent one another; and

said line conductors are tapered in the regions of said divergence, increasing in thickness as said line conductors diverge from one another.

8. A directional coupler as in claim 7 wherein each of said dielectric compensating means comprises a dielectric element having a varied thickness and being thickest in the area of greatest line conductor tapering to help pro vide said coupler with a substantially uniform transmission line impedance.

9. A directional coupler as in claim 8 wherein said dielectric elements are substantially annular-shaped elements supporting said first and second line conductors at their diverging portions.

10. A directional coupler as in claim 9 wherein said planar reference conductors are spaced a greater distance from said line conductors in the coupling region where saidline conductors are parallel and equidistant than in the regions of divergence to improve the uniformity of transmission line impedance of said line conductors in the coupling region.

References Cited R. M. Barrett: Etched Sheets Serve as Microwave Components, Electronics, June 1952.

HERMAN KARL SAALBACH', Primary Examiner PAUL L. GENSLER, Assistant Examiner U.S. Cl. X.R. 33334, 84 

